(1) Field of the Invention
This invention relates to a process for producing a phenyl ester in a high yield and stably by allowing benzene, a carboxylic acid and a molecular oxygen to react with each other in the presence of a specific palladium catalyst. It also relates to the specific palladium catalyst used in the process for the production of a phenyl ester.
(2) Description of the Related Art
A process for producing a phenyl ester by allowing benzene, a carboxylic acid and molecular oxygen to react with each other in the presence of a catalyst is well known. Proposals have been made wherein the reaction is conducted in the vapor phase or liquid phase using a noble metal catalyst. Palladium is most popularly used as the main ingredient of the noble catalyst, and some proposals also have been made wherein a co-catalyst comprising a metal, which exhibits by itself no catalytic activity for the specified reaction, is used in combination with the noble catalyst.
As examples of the process using a metal catalyst, there can be mentioned a process using at least one metal selected from palladium and platinum, described in Japanese Examined Patent Publication (hereinafter abbreviated to “JP-B”) S46-33024, a process using a combination of at least one metal selected from palladium and platinum with elemental bismuth or tellurium, described in JP-B S48-18219. Further, a process using a catalyst comprising a combination of palladium or a palladium compound with at least one compound comprising a metal selected from cadmium, zinc, uranium, tin, lead, antimony, bismuth, tellurium and thallium, and in the presence of nitric acid is described in JP-B S55-15455.
A catalyst system comprising palladium and antimony, and an alkali metal salt as an activation promoter is described in JP-B 56-21463, Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) S52-27089, JP-A 552-77892 and JP-A S52-130494.
Further, as examples of the process using a metal compound as catalyst, there can be mentioned a process using a catalyst comprising a combination of at least one metal compound selected form an oxide, a hydroxide, an acetate or a nitrate of a metal selected from platinum, palladium, rhodium, ruthenium, iridium or osmium, with at least one alkali metal nitrate (JP-B S50-34544), a process using a combination of (a) at least one member selected from metallic palladium and palladium compounds, with (b) at least one compound selected from nitric acid, nitrous acid and metal salts of these acids, or a combination of (a), (b) with (c) at least one metal salt of a carboxylic acid (JP-A S48-4439, and a process using a combination of palladium acetate with antimony acetate, and at least one metal acetate, the metal of which is selected from chromium, nickel, manganese and iron (JP-B H2-13653).
The processes for allowing benzene, a carboxylic acid and molecular oxygen to react with each other in the liquid phase using a palladium catalyst or a palladium compound catalyst to produce a phenyl ester have a problem such that palladium metal is dissolved in the raw material liquid, and the catalytic activity is reduced with time. Palladium is expensive and thus the above processes are costly. If a step of recovering palladium is conducted, the production processes become complicated. Further, the operation of compensating the decrease of catalytic activity with time is troublesome and not advantageous from an industrial point of view.
In a process using a metal salt catalyst soluble in a reaction liquid, a step of recovering the metal salt must be conducted. Further, a problem arises such that, for example, a palladium salt is used, palladium metal is liable to be deposited on the inner wall of a reactor during the reaction, and this also leads to reduction of catalytic activity with time and loss of palladium.
A process comprising a liquid phase reaction using as a catalyst a combination of palladium with bismuth and/or lead wherein a soluble bismuth compound and/or a soluble lead compound is additionally incorporated in the reaction system is described in JP-A S63-174950. In this process, the soluble bismuth compound and/or the soluble lead compound prevents dissolution of metallic bismuth or lead supported on the palladium catalyst, and thus, dissolution of the main catalyst ingredient, i.e., palladium can be suppressed and the reduction with time of catalytic activity can be minimized. This process has a problem such that the amount of the soluble bismuth compound and/or the soluble lead compound incorporated is large, and the soluble compounds must be recovered as a crystal at the step of separating and purifying a phenyl ester, which leads to complication of the production process.